A known control strategy for an electromechanical device comprises the controlled delivery of electric pulses to an electromechanical actuator of the device. One example of such a control strategy comprises the delivery of a pulse-width-modulated (PWM) waveform to a device. Such a control strategy has heretofore been proposed for the control of certain solenoid-actuated devices used in motor vehicles that are powered by internal combustion engines. Examples of such devices include emission control valves like electric exhaust gas recirculation (EEGR) valves and canister purge solenoid (CPS) valves. The ensuing disclosure of the inventive principles will be presented in connection with control of a CPS valve in an on-board evaporative emission control system for an automotive vehicle.
A known on-board evaporative emission control system comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of the fuel tank by the volatilization of liquid fuel in the tank and a CPS valve for periodically purging collected vapors to an intake manifold of the engine. The CPS valve comprises a solenoid actuator that is under the control of a microprocessor-based engine management system.
During conditions conducive to purging as determined by the engine management system on the basis of various inputs to it, evaporative emission space that is cooperatively defined by the tank headspace and the canister is purged to the engine intake manifold through the CPS valve, which is fluid-connected between the canister and the engine intake manifold. The CPS valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw volatile fuel vapors from the canister for entrainment with the combustible mixture passing into the engine's combustion chamber space at a rate consistent with engine operation to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
A known CPS valve comprises a movable valve element that is resiliently biased by a compression spring against a valve seat to close the valve to flow when no electric current is being delivered to the solenoid. As electric current begins to be increasingly applied to the solenoid, increasing electromagnetic force acts in a sense tending to unseat the valve element and thereby open the valve to fluid flow. This electromagnetic force must overcome various forces acting on the mechanical mechanism before the valve element can begin to unseat, including overcoming both whatever static friction (stiction) is present between the valve element and the seat, as well as the opposing spring bias force. Once the valve element has unseated, the valve element/valve seat geometry also plays a role in defining the functional relationship of fluid flow rate through-the valve to electric current supplied to the solenoid coil. Furthermore, the extent to which a given valve possesses hysteresis will also be reflected in the functional relationship.
When the valve element comprises a tapered pintle that is selectively positioned axially within a circular orifice which is circumscribed by the valve seat, a well defined flow rate vs. pintle position characteristic can be obtained. However, certain geometric factors present at the valve element/valve seat interface may prevent this characteristic from becoming effective until the valve element has unseated a certain minimum distance from the valve seat. Accordingly, each graph plot of fluid flow rate through the valve vs. electric current supplied to the solenoid coil may be considered to comprise distinct spans: a short initial span that occurs between valve closed position and a certain minimum valve opening: and a more extensive subsequent span that occurs beyond a certain minimum valve opening.
One specific type of CPS valve comprises a linear solenoid and a linear compression spring that is increasingly compressed as the valve increasingly opens. It is sometimes referred to as a linear solenoid purge valve. Such a valve can provide certain desirable characteristics for flow control. By itself, a linear solenoid possesses a force vs. electric current characteristic that is basically linear over a certain range of current. When a linear solenoid is incorporated in an electromechanical device, the overall electromechanical mechanism possesses an output vs. electric current characteristic that is a function of not just the solenoid, but also the mechanical mechanism to which the solenoid force is applied. As a consequence then, the output vs. electric current characteristic of the overall device is somewhat modified from that of the linear solenoid alone, and the same is true in the case of a PPS valve.
While a CPS valve that incorporates both a linear solenoid and a tapered pintle valve element which is selectively positionable axially within a circular orifice that is circumscribed by the valve seat can exhibit a desired fluid flow rate vs. pintle position characteristic, such characteristic may not become effective until after the pintle has opened a certain minimum amount because of geometric factors at the pintle/seat interface, as noted earlier. Accordingly, each graph plot of fluid flow rate through the valve vs. electric current applied to the solenoid coil may be considered to comprises the spans referred to above, namely, a short initial span that occurs between valve closed position and a certain minimum valve opening, and a more extensive subsequent span that occurs beyond a certain minimum valve opening.
Generally speaking, a linear solenoid purge valve that is not pressure compensated is graphically characterized by a series of graph plots of fluid flow rate vs. electric current, each of which is correlated to a particular pressure differential across the valve. Each graph plot may be characterized by the aforementioned short initial span and the more extensive subsequent span. Within the latter span of each graph plot, one especially desirable attribute is a substantially constant relationship between incremental change in an electric control current applied to the solenoid and incremental change in fluid flow rate through the valve. Within the former span, incremental change in fluid flow rate through the valve may bear a substantially different relationship to incremental change in an electric control current applied to the solenoid, once again because of the valve element/valve seat interface geometry.
In one such linear solenoid purge valve, a certain minimum electric current is required before the valve begins to open. As the valve begins to open, each fluid flow rate vs. electric current graph plot follows the relatively short initial span where a small incremental change in electric current will result in an incremental change in flow that is much larger than the incremental change that would result were the valve instead operating within the span where the valve has opened beyond the certain minimum and incremental change in flow through the valve bears a substantially constant relationship to incremental change in electric current.
Electric current to the solenoid coil can be delivered in various ways. One known way is by applying a pulse width modulated D.C. voltage across the solenoid coil. In choosing the pulse frequency of the applied voltage, consideration should be given to the frequency response characteristic of the combined solenoid and valve mechanism. If a pulse frequency that is well within the frequency response range of the combined solenoid and valve mechanism is used, the mechanism will faithfully track the pulse width signal. On the other hand, if a pulse frequency that is well beyond the frequency response range of the combined solenoid and valve mechanism is used, the mechanism will be positioned according to the time average of the applied voltage pulses. The latter technique may be preferred over the former because the valve mechanism will not reciprocate at the higher frequency pulse width modulated waveform, but rather will assume a position corresponding to the time averaged current flow in the solenoid coil. Under the former technique, the mechanism would by contrast experience significant reciprocation as it tracks the lower frequency waveform.
While PWM control may therefore be a desirable technique to control a solenoid-actuated valve over a range where the ratio of incremental change in fluid flow rate to incremental change in average solenoid current is substantially constant, accurate control may be more difficult to achieve over a range where such a characteristic does not exist.
Accordingly, a need exists for further device in certain aspects of control strategy for pulse-operated electromechanical devices such as fluid valves. Devices are particularly significant for automotive vehicle emission control valves because such valves may be required to perform under diverse vehicle operating conditions. For a CPS valve, purging of volatile fuel vapor to the intake manifold when the engine is idling may be quite difficult to accurately control.